Chain propagation ethers

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

Rate Constants of Chain Propagation and Termination in Oxidized Ethers... 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

Let us compare the competition of intermolecular and intramolecular chain propagations in oxidized dibenzyl ether. The rate constant A p(R02 + RH) = 95 L mol-1s-1 (T = 303 K, Table... 

The estimation of the reactivities of the free ions and ion pairs directly in the polymerization reaction of phenylglycidyl ether under the action of dimethylbenzylamine in the presence of isopropyl alcohol at 343 K 15l) gave k = 5.6 1 mol-1 s-1 and k = 0.71 mol1 s 1. The values of the bimolecular rate constants are given here considering the fact that the activated molecules of the monomer (its complexes with alcohol) take part in the chain propagation reaction. 

Small amounts of polar solvents such as tetrahydrofuran, ether, dioxane and triethylamine have been shown to break down the association of organo-lithium compounds in non-polar solvents, and to greatly increase the rate of chain initiation. In polar solvents, therefore, one expects rapid initiation and a polymerization rate essentially determined by the rate of chain propagation of solvated ion-pairs. 

A clear consensus47 156>166) has emerged which indicates that various extents of ether complexation with active centers can reduce their reactivity in the chain propagation event. If cation-monomer coordination is important, the presence of ether in the coordination sphere might be expected to lead to less monomer interaction with a subsequent reduction in polymerization reactivity. Clearly, there is a need for further work, experimental and theoretical, on this topic. 

The tin additive is present in the liquid state under the conditions of the present experiments. It has a smaller inhibiting effect than iron on the reactivity of the phenoxy and benzyl ethers. Two explanations are plausible. Hydrogen dissolved in the tin may react with the benzyl and phenoxy radicals which are the chain propagators and remove them from the system. The rate of bond cleavage is therefore lowered. Alternatively, tin may promote radical recombination reactions. By either route the tin would be acting to inhibit propagation reactions. 

The reaction probably involves a cationic chain reaction involving hydride transfer, and indeed the rate-limiting step in the oxidation of trityl benzyl ether was shown to be a chain-propagating step (equation I). 

Penczek and Kubisa developed a new cationic polymerization technique for cyclic monomers in which the chain propagation involves the reaction of a protonated (activated) monomer molecule with a nucleophilic site in the neutral growing macromolecule. This so-called activated monomer (AM) polymerization is depicted mechanistically in Scheme 58. According to this mechanism, when the polymerization of an oxirane (a cyclic ether) is carried out in the presence of... 

In contrast to catalysis by complexes Ni(II)(L )2 (L =acac, enamac ) with 18K6 in reaction of ethylbenzene oxidation catalyzed by Ni(II)(L )2 in the absence of crown-ethers additives increase of initial rate of oxidation is coimected mainly with participation of catalyst at stage of chain propagation. At that under catalysis by Ni(0,NH>2 complex the value is twice as much than under catalysis by Ni(0,0)2. at the same time the rate of chain initiation almost in order exceeds w in oxidation reaction catalyzed by Ni(II)(acac)2. As it obvious, presence of donor NH-groups in chelate group of nickel complex promotes significant increase of role of activation reaction of molecular oxygen in catalysis mechanism [33]. 

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